Effect of double-diffusive heat transfer on thermal conductivity of porous sintered ceramics: Macrotransport analysis

Abstract

Heat conduction in composite materials has been traditionally considered as occurring via conductance through the solid phase and also in pores containing gas. In this paper these mechanisms are considered together with thermal effects associated with motion of mass species – impurities, or lattice defects, normally present in all sintered ceramic materials. Diffusion and reversible segregation and desegregation of these species (from solid bulk to pore surfaces and back), is induced by an externally applied temperature gradient and accompanied by heat release and absorption on pore surface. A general method is developed which allows calculation of the effective thermal conductivity of ceramic materials with complicated microstructure subject to the segregation–diffusion processes. The model is based on the macrotransport analysis of heat transfer in composite materials, generalized to include the thermal effect associated with reversible segregation and diffusion in the grain boundary region. The method is illustrated by calculation of the effective thermal conductivity of ceramics containing chains of pores in the grain boundaries. The physical circumstances, at which segregation–diffusion processes significantly affect thermal conductivity dependence on temperature measured in vacuum, are outlined.